Females of all animal species produce larger reproductive cells (called "gametes") than males. Eggs are bigger than sperm, and this difference in material investment is far more substantial than most of us appreciate. Humans are rather ordinary in this respect, but we're a good place to start. The female egg is the largest cell in the human body. It measures almost a fifth of a millimeter across—about the size of a period (.) on this page—and it's just visible to the naked eye. Sperm are the smallest cells in the body, and a hundred thousand could fit into the volume of a single egg.

Emlen, Douglas J, Animal Weapons_ The Evoluti…, loc. 151